Extremophiles

Living organisms that can live in the world's extreme environments
Grand Prismatic Spring from Overlook
Grand Prismatic Spring from the Overlook

Extremophiles are microscopic organisms that thrive in extreme environments. In Yellowstone, these conditions can include extreme heat or an altered pH—either very low pH (acidic), very high pH (alkaline), or a combination of heat and altered pH.

The environments of extremophiles would kill other organisms. However, if their environment changed to conditions we would consider more hospitable, the extremophiles would perish.

Extremophiles are a type of microorganism, a small living thing that must be viewed with a microscope. However, colonies, or large groups of microorganisms living together, are visible in many of Yellowstone’s thermal features.

Close in to the bacteria mat below Sapphire Pool.
There is a large flat area of runoff below Sapphire Pool. The runoff channel with the hottest water is green for a considerable distance from the pool. This orange mat is on higher ground, by a couple of inches, and is still hot. It has an intriguing display of bacteria and geyserite growing together. Looking closely, you may find the ephydrid flies walking on the water, eating bacteria.
Whirligig Geyser in Norris Geyser Basin
Norris Geyser Basin is predominantly acidic. Meaning it supports acidophiles, actually thermoacidophiles, because the water is also hot.

Extremophiles in Yellowstone fall into two categories:

  • Thermophiles are microorganisms that live in extremely hot environments.
    • Thermophiles thrive in temperatures above 113° F (45° C).
    • Hyperthermophiles thrive in temperatures between 176° F (80° C) and 235° F (113° C).
  • Some extremophiles require hot conditions but also thrive in environments with altered pH.
    • Acidophiles thrive in environments with a pH level of 1 to 5. So, the acidophiles in Yellowstone are actually thermoacidophiles—they thrive in hot and acidic environments.
    • Alkaliphiles live and grow in environments with a pH level of 9 to 11.

Let us review what pH measurements mean: pH is a scale of acidity and alkalinity ranging from 1 to 14, with seven indicating neutral, neither acidic nor alkaline. Any pH below seven is considered acidic; anything above is alkaline or basic. A substance with a pH close to 1 would be a powerful acid; a pH close to 14 would be highly alkaline.

To better appreciate the impact of pH, comparing the conditions in Yellowstone’s thermal features to those tolerated by humans may be helpful. In the human body, our normal pH is 7.35 to 7.45. A pH above 7.8 or below 6.8 is generally fatal. Our pH may vary slightly from 7.35 to 7.45 during the day, but only by minute amounts.

pH Scale
Here is an example of the pH scale, with items that match along it. Image by Freepik.
Sulphur Caldron
Sulphur Caldron is south of Hayden Valley on the park's east side. Sulphur Caldron is one of the most acidic features in the park, with a pH of around one.

Other extremophiles found throughout the world include:

  • Barophiles are microorganisms that survive under tremendous pressure.
  • Halophiles are aerobic (oxygen-using) microorganisms that live and grow in high-salinity environments.
  • Psychrophiles thrive in extreme cold. They grow best in environments between 14° and 68° F (-10° and 20° C).
  • Xerophiles live in arid environments.

“Normal” microorganisms are called mesophiles. They grow best at temperatures between 50° and 122° F (10° and 50° C).

Runoff from Excelsior Geyser
Excelsior Geyser runoff channel. The Geyser itself is 132°F (55.56°C). The runoff is cooler as it gets away from the heat source. Notice the green water is in a lower channel than the ground around it. The orange color is elevated above the green, and the color of the Extremophiles is darker because the water is cooler. The water is cooler because it runs slowly, not deep, and is exposed more to the wind.